There are work machines that have a cooling fan to cool fluids of the machine. In some cases, the hydraulic circuit for driving the fan has a proportional pressure-relief valve to control the speed of the fan. Contamination has become lodged in the valve causing the spool thereof to stick open, preventing the fan from increasing to a desired speed of rotation and thereby causing the machine to over-heat due to insufficient cooling by the cooling fan. This has resulted in machine downtime and decreased productivity. The contamination has been dislodged from the valve by human intervention to restore valve functionality.